Breads, cookies, vegetables, fermented foods such as fermented soybeans and kimchi, etc. have been conventionally sold in a state wrapped in paper or plastic bags. Though paper bags have high air permeability and moisture permeability, they are disadvantageous in not permitting their contents to be seen. On the other hand, plastic bags permit contents to be seen, but they are disadvantageous in not having sufficient air permeability and moisture permeability, and extremely deteriorating the flavor and texture of foods.
To obtain a plastic film permitting contents to be well seen, while having high air permeability and moisture permeability, apparatuses for forming large numbers of fine pores in a plastic film are known. For example, JP 6-71598 A discloses an apparatus for producing a microporous film comprising a means for supplying a long plastic film, a pattern roll having large numbers of high-hardness, fine particles having sharp edges and Mohs hardness of 5 or more fixed to its rolling surface, an anvil roll (metal roll) having a flat rolling surface and rotatable in an opposite direction to that of the pattern roll, a pressure-adjusting means disposed near both ends of either one roll for adjusting a pressing force to the long plastic film, and a means for applying high voltage to the pattern roll, either one or both of the pattern roll and the anvil roll being movable in their arrangement direction. The pattern roll and the anvil roll are arranged in parallel, and the long plastic film passing through a gap therebetween is provided with large numbers of fine pores by large numbers of high-hardness, fine particles on the pattern roll.
However, when large numbers of fine pores are formed in a plastic film (not shown) as thick as about 8-100 μm passing through a gap between the pattern roll 10 and the anvil roll 20, a large load is applied to the pattern roll 10 and the anvil roll 20, so that both rolls 10, 20 tend to be bent, resulting in a gap G, which is wider in a width-direction center portion than in both side edge portions, as shown in FIG. 13. Fine pores formed by an uneven gap G have different characteristics (opening diameters, depths, areal density, etc.) between a width-direction center portion and both side edge portions of the film, failing to obtain a microporous plastic film having transversely uniform air permeability.
To prevent the bending of a pattern roll 10 and an anvil roll 20, it may be considered to arrange backup rolls above the pattern roll 10 and/or under the anvil roll 20. However, it has been found that such backup rolls fail to prevent the bending of the pattern roll 10 and the anvil roll 20 sufficiently. In addition, because of large numbers of high-hardness, fine particles attached to a rolling surface of the pattern roll 10, a backup roll arranged above the pattern roll 10 should be a soft-surface rubber roll, etc., failing to sufficiently prevent the bending of the pattern roll 10.
JP 6-328483 A discloses an apparatus for calendaring a film of a thermoplastic polymer or rubber by six rolls arranged in substantially parallel, a sixth roll being arranged just under first to fifth rolls; and comprising a crossing means for inclining the fifth roll by a small angle. However, the angle of the fifth roll inclined by the crossing means is constant, and the calendaring apparatus does not have a driving means for obtaining a desired inclination angle. Accordingly, even if this calendaring apparatus were used to form fine pores in a plastic film, fine pores having various sizes and distributions cannot be formed uniformly in a width direction.
To solve the above problems, the present inventor previously proposed by Japanese Patent 6125707 an apparatus for producing a microporous plastic film, which comprises a pattern roll rotatably supported by a pair of laterally arranged stationary frames; an anvil roll movable up and down along a pair of laterally arranged movable frames, such that the anvil roll is brought into contact with the pattern roll via a plastic film; a conveying means for passing the plastic film through a gap between the pattern roll and the anvil roll; a first driving means for rotating a pair of the movable frames; a second driving means each mounted to each of the movable frames for moving up and down the anvil roll; a third driving means for rotating the pattern roll; and a fourth driving means for rotating the anvil roll; in a state where the anvil roll is inclined to the pattern roll in a horizontal plane by the operation of the first driving means, the plastic film passing through a gap between the pattern roll and the anvil roll, so that the high-hardness, fine particles form large numbers of fine pores in the plastic film.
The above apparatus can form fine pores having various sizes and distributions uniformly in a width direction, but it has been found that it suffers in actual production the following problem: A long microporous plastic film having insufficient width-direction uniformity of fine pores is inevitably formed until reaching the optimum inclination angle of the anvil roll, because the optimum inclination angle of the anvil roll to the pattern roll is determined by repeating slight change of the inclination angle of the anvil roll and the measurement of transverse distribution of fine pores. Accordingly, an apparatus capable of quickly making uniform the width-direction distribution of fine pores is desired.
In the rolling of a steel sheet S as shown in FIG. 14, a pair of work rolls 210, 220 are inclined, with backup rolls 211, 221 arranged for both work rolls 210, 220, there is no inclination angle between each work roll 210, 220 and each backup roll 211, 221. Accordingly, a slanted force is applied to the steel sheet S in the structure as shown in FIG. 14. Though the steel sheet S having sufficient strength can withstand the slanted force, a lower-strength plastic film likely suffers wrinkling and rupture, failing to conduct quick inclination of the work rolls 210, 220. Such simple arrangement of backup rolls on both sides of inclined pattern roll and anvil roll cannot achieve quick optimization of the inclination angle of one work roll, failing to produce a microporous plastic film efficiently.
Dry foods such as instant coffee, powdered milk, tea, etc. are contained in bags of aluminum-vapor-deposited plastic films, etc. having good sealability for protecting them from oxygen and moisture. The aluminum-vapor-deposited plastic film is composed of, for example, a high-strength polyethylene terephthalate (PET) film, a print layer, a vapor-deposited layer, and a heat-sealing layer. Most aluminum-vapor-deposited plastic film bags are provided with notches for tearing.
However, because of a vapor-deposited aluminum layer, a heat-sealing layer and a print layer laminated with a PET film, it is sometimes not easy to tear the aluminum-vapor-deposited plastic film even with a notch. Particularly a heat-sealed portion of the aluminum-vapor-deposited plastic film bag is as thick as two times, tearing from the notch is often stopped by the heat-sealed portion.
In view of the above circumstances, as a film easily tearable from any point without a notch, the present inventor previously proposed by JP 7-165256 A, an easy-to-tear plastic laminate film comprising a porous film of polyester, nylon or oriented polypropylene having penetrating pores or unpenetrating pores having an average opening diameter of 0.5-100 μm at a density of 1000/cm2 or more on the entire surface, and a heat-fusible polymer film laminated to one surface of the porous film. However, this easy-to-tear plastic film having penetrating pores cannot be used for applications which must have completely no permeation of oxygen and moisture.
As a plastic film having only unpenetrating fine pores, JP 10-193454 A discloses a tubular film as thick as 5-150 μm, which is made of a polyolefinic resin composition containing inorganic fillers, one or both of its inner and outer surfaces being corona-discharged, and at least part of it being embossed. Embosses are as deep as ½ to 1/10 of the film thickness (measured according to JIS B 0601), and usually as wide as 0.5-300 mm. However, these embosses are too large, failing to make the tubular film easy to tear, and deteriorating the appearance of the tubular film. If fine embosses were to be formed for easiness to tear, an extremely expensive emboss roll having large numbers of fine projections would have to be used, inevitably resulting in an expensive embossed film.
Also, even when only unpenetrating fine pores are formed on a plastic film, it suffers uneven distribution of unpenetrating pores. To solve this problem, a pressing force applied to the plastic film by a pattern roll and an anvil roll should be uniform in a transverse direction, but the structure described in Japanese Patent 6125707 is not necessarily optimum to quickly make the pressing force uniform in a transverse direction.
U.S. Pat. No. 5,839,313 discloses, as shown in FIG. 15, a rolling mill comprising a pair of work rolls 301, 302, backup rolls 303, 304 each pushing each work roll 301, 302, and an immediate roll 305 disposed between the upper work roll 301 and the upper backup roll 303, a pair of work rolls 301, 302 and a pair of backup rolls 303, 304 being all in parallel, and only the immediate roll 305 being inclined to a pass line. The rolling mill of U.S. Pat. No. 5,839,313 can prevent the transverse deviation (strip walking) of a thin metal strip, but it is difficult to make this rolling mill smaller because it has a structure in which all rolls are supported by the laterally arranged frames.